In some industrial processes such as power generation, steam generation, and thermally driven chemical processing, heat can be provided directly or indirectly by the combustion of high-energy-content (HEC) fuels, such as propane or natural gas.
Emissions from landfills and other sources of gas containing volatile organic compounds (VOCs) are considered pollutants. These waste streams often contain too little fuel to sustain combustion on their own. Some methods of disposing of VOC-containing waste streams use thermal oxidizers of the following types: (1) Fired- or supplemental-fired thermal oxidizers, (2) Catalytic thermal oxidizers, (3) Oxidizers with heat recovery, and (4) Regenerative thermal oxidizers (RTOs).
Fired- or supplemental-fired thermal oxidizers can include a burner, a residence chamber, a mixing chamber, and an exhaust stack. FIG. 1-1A illustrates a configuration wherein an air-fuel mixture 6 is provided to the burner 2 to create a continuous flame and the waste stream 7 is introduced into the flame and continues to oxide as the hot gases pass through the mixing chamber 3 and residence chamber 4. If the waste stream 7 is within flammability limits, it may be directly combusted in the burner 2 in place of the air-fuel mixture 6. The mixing chamber 3 is required if the waste stream and burner are separately supplied. The residence chamber 4 provides enough time to complete the oxidative chemical reactions. The exhaust stack 5 conveys the products of oxidation to the atmosphere.
Catalytic oxidizers, as shown in FIG. 1-1B, avoid the creation of thermal NOx by keeping the oxidation reaction temperature low. A waste stream 7 containing VOCs is provided into a catalytic reaction chamber 8 having a large internal surface area coated with a catalyst. Catalytic materials include noble metals such as platinum, palladium, and iridium as well as, for certain VOCs, copper oxide, vanadium, and cobalt. The concentration of VOCs in the waste stream 7 must be low enough that the reaction temperatures will not exceed the catalyst maximum use temperature. The waste stream 7 typically has to be heated to a specific temperature range appropriate for the catalytic reactivity.
The use of a recuperator 9, as shown in FIG. 1-1C, can reduce the operating costs of fired thermal oxidizers and catalytic oxidizers. The exhaust from the reaction chamber 1, which may be by way of example either of the systems of FIG. 1-1A or 1-1B, is supplied to a high-temperature recuperator 9 to heat either the VOC-laden waste stream 7, as shown in FIG. 1-1C, or the separate combustion air-fuel mixture if supplied separately, as shown in FIG. 1-1A. Use of a recuperator 9 can reduce or eliminate the need for supplemental fuel to heat the reactants to their oxidation temperature.
Lastly, RTOs can be used to oxidize VOCs. In an RTO, heat is stored on an intermediate heat sink material, usually a ceramic solid, for recovery during an alternate cycle. The cycle uses heat from a previously heated flow to preheat the VOC-laden waste stream to a higher temperature. If the temperature is sufficiently high, oxidation will take place due to autoignition, as discussed in greater detail later in the present disclosure. If the temperature is not high enough, supplemental firing from another fuel and air source may be required. The higher-temperature exhaust is then conveyed through a colder heat sink to capture the energy.
There are different approaches to achieve the cycling of the heat exchange material. FIGS. 1-1D illustrates a system using two regenerative oxidizers. In the depicted configuration, the waste stream 7 is introduced into hot regenerative oxidizer #1. The waste stream is heated as it passes through regenerative oxidizer #1, thereby incrementally cooling the heat sink material with the oxidizer #1 starting at the inlet. After the waste stream 7 autoignites, the hot exhaust gas exits from the oxidizer #1 and is provided to the inlet of oxidizer #2, thereby “regenerating” the stored thermal energy in the heat sink material in oxidizer #2. The oxidized waste stream cools as it passes through oxidizer #2. When oxidizer #2 is sufficiently heated, the system is reconfigured such that the flow from the waste stream 7 is provided to the inlet of oxidizer #2 and the exhaust from oxidizer #2 is provided to the inlet of oxidizer #1 to regenerate oxidizer #1. The process cycles between the two configurations so that the oxidizer that was previously cooled while heating the waste stream 7 is heated, and visa-versa. Some RTO designs make use of rotating hardware to variably change the flow streams between cycles or to move the regenerative oxidizers between cycles. Another approach is to use a single regenerative oxidizer but to reverse the flow direction for each cycle. One end of the oxidizer will be preheating while the other end is capturing heat after the oxidative reaction. The reversing of flow direction is necessary because the end of the oxidizer proximal to the inlet cools to the point where it can no longer heat the incoming waste stream 7 to a temperature that will initiate the reaction.